Pre-insulated pipes are known in which an insulation material is interposed between inner and outer pipe members. However, the formation of joints connecting two pipes has proven to be difficult and involves the need for producing a foam material in the field.
Although numerous pipe joints are knwon in the art, as for example in my earlier U.S. Pat. Nos. 3,606,402; 3,606,403; 3,860,742; 4,032,708; as well as in other patents such as U.S. Pat. No. 3,913,954, these patents are only concerned with pipes of singular tube construction.